Pre 2002 stereos don't have an aux socket at the back. I had one of the interfaces in the link below in my early Cooper. Basically plugs in into the CD changer socket at the back of the stereo, (providing you're not using it for CD changer). It connects to the mp3 player via the headphones socket. All functions are operated through the mp3 but obviously the volume can be adjusted via the stereo.
NewMINIStuff.com 
